In India, sugar is manufactured by a conventional double sulfitation process from sugarcane. FIG. 1 shows a flowchart of the double sulfitation process. The first step of the process includes steps of fibrillation of cane by passing canes through cane leveler, cutters, and fiberizers/shredders. This step is also known as cane preparation, which takes generally one minute to reach to further step.
In the second step, the prepared sugarcane is fed to cutter and then chopped and fed along with juice through a series of milling tandems (herein after referred to as mill). This step is known as milling where juice is extracted from the bagasse. Generally, milling comprises four or five mills in series. Normally, hot water is used in the last mill for maximum extraction of the juice from bagasse and juice collected is added in the previous mill. During milling, aqueous solutions of Chlorine, Iodine, Quaternary ammonium compounds, corbomate or dithiscorbonate based may be added for cane sanitation.
The juice obtained from mills is then passed through a series of juice heater for better reaction of juice with lime. In the 1st juice heater, juice is heated at about temperature of 70° C. However, heating of juice changes microbiology of the juice, where microbes having capacity to grow at a high temperature initiates their growth and microbes growing at normal temperature either die or remain in dormant stage till favorable conditions return.
The heated juice is then treated with milk of lime to raise pH above 9 or desired level to facilitate precipitation of inorganic impurities which is immediately followed by treatment with SO2 gas released from sulfur burner in a sulfitation tower to reduce to a pH of juice to about 7.0 and decolorization. This is a FIRST SULFITATION step. Phosphoric acid may be added to the juice if sugarcane does not have sufficient phosphate content. The sulfited juice is passed through heater again to raise temperature to about 105° C. The heated juice is then sent to settling tank/clarifier (herein after referred to as Dorr settler). Flocculants and allied chemicals are added to promote flocculation and settling of mud and impurities in the Dorr settler. Juice overflowing from Dorr is clear and free from suspended impurities, which is then send through to series of evaporators to boil off water for further concentration to obtain concentrated juice of about 60% solids (brix) which is called as syrup. This syrup is again treated with SO2 gas to decolorize the syrup and is sent to further concentration and crystallization in crystallizers and pans. This is SECOND SULFITATION step. This sulfated syrup has generally acidic pH about 5.0-5.5 and remaining process is thus carried with acidic pH. The liquor containing solid sugar from Pans dropped in centrifuge where sugar is separated from mother-liquor (molasses) by centrifugal force and is washed with steam and/or hot water. Sugar separated is the final product which is bagged after drying on through hoppers and silo. Petrochemical base or polymer base such as DOSS, NP10, surfactants as viscosity reducers may be used to reduce viscosity as a pan aid agent. This sugar contains sulfur dioxide, lead, conductivity ash, reducing sugar and moisture along with sucrose. The specifications of such plantation white sugar are as follows:
IS: 5982-2003 Requirements for Plantation White SugarCharacteristicsRequirementMoisture, percent by mass, Max.0.10Sucrose, percent by weight, Min.99.5Reducing sugars, present by weight, Max.0.10Colour in ICUMSA units, Max.150Conductivity ash %, Max.0.1Sulphur dioxide, mg/kg Max.70Lead, ppm, Max.5.0
The sulfur treatment on juice and syrup is carried out mainly for decolorizing juice and syrup. The sulfur treatments along with mill sanitation as described above also help to reduce microbial activity. However, as sulfitation and mill sanitation only reduces microbial activity to certain extent and sugar loss due to microorganisms/microbes takes place during the process resulting in loss in the final yield of the sugar. Further, in the last stage of the process, syrup has an acidic pH which results in further loss of the sugar. Moreover, the sugar produced by this method contain significant amount of sulfur (SO2) content i.e. 50-70 ppm, significant level of impurities and has acidic pH, therefore, not good for human health also. Furthermore, sugar produced by above double sulfitation process looses its color in storage as bleaching effect of sulfur (SO2) reduces with time. Sugar with higher SO2 and lower pH has limited keeping quality or in other words, quality of sugar deteriorates significantly during storage, that is, the sugar produced by this process has lower shelf life.
Further, day by day cost of Sulfur is also increasing resulting in the increasing the expenses of the double sulfitation process.